A grating is an optical device formed by a plurality of fine structures which is arranged orderly. When an energy beam, such as a light beam is incident on the gating, the light illuminated on the grating surface of the grating can be diffracted to different angles predicted by the basic grating equation as follows:d(sin(α)+sin(β))=mλ  (1)
where d denotes the grating pitch, α the incident angle, β denotes the diffraction angle, m denotes the diffraction order, and λ denotes the wavelength of the diffracted light. Therefore, the grating can serve as an optical filter.
Please refer to FIG. 1, which shows the structure of a typical diffraction grating. As can be known from FIG. 1, when an energy beam is irradiated into a grating plane 15′ at an incident angle, the diffracted energy beam with the diffraction order m and the wavelength λ would be detected at the diffraction angle β. Meanwhile, as can be predicted from the equation (1), further diffracted energy beams, such as the diffracted energy beam with diffraction order 2m and the wavelength λ/2 and other diffracted energy beam in case the product of the diffraction order and the wavelength remains the same, can be detected at the same diffraction angle β. Therefore, the diffracted energy beam detected at the diffraction angle β includes an overlapped energy beam in successive orders and multiple wavelengths.
On the other hand, it also can be seen from the equation (1) that the wavelength of the diffracted energy beam would be modulated with the change of the grating pitch. With such modulation, a monochromatic energy beam can be separated from an energy beam with a continuous spectrum. Therefore, the grating as FIG. 1 can be used for the various dispersive optical systems, such as the spectrometer, the optical communication component, and the resonance cavity of laser, etc.
However, a disadvantage in such grating as shown in FIG. 1 is that the gap width c between two teeth increases with the grating pitch d. As a result, only a small portion of the incident light (or energy beam) can be diffracted by the grating and contributes to the light collected by the detector. Therefore, the diffraction efficiency decreases with the grating pitch d.
Furthermore, please refer to FIG. 2(A), which shows a top view of a further typical grating structure. The main structure of the grating 100′ is formed on a substrate and manufactured with the MEMS-based process. Furthermore, the main structure of the grating 100′ is manufactured in a form of a resilient set, such as, a spring set, in order to form a pitch-tunable grating. As can be seen from FIG. 2(A), the grating 100′ is formed by a resilient set 10′, on which a grating plane 15″ is formed on the cross section plane AA′ of the side wall of the resilient set 10′. The grating plane, which can be seen from FIG. 2(B), includes a plurality of support units 101′ and gaps 102′ arranged orderly and alternately. A grating pitch d′ is formed by each support unit and gap width. Because the grating 100′ is formed by the resilient set 10′, the grating pitch d is tunable if a deformation of the resilient set 10′ occurs.
It can be seen from FIG. 2(A), the smallest pitch of the grating is the same as the width of the support unit 101′. Therefore, the attainable grating pitch is constrained by the width of the support unit 101′, which is inevitably much broader than the minimum line-width of the grating structure, and thus resulting in much smaller modulation range of the grating pitch d.
Accordingly, it is the first aspect of the present invention to provide a novel grating structure, with which the broader modulation range of the grating pitch is achieved. Furthermore, it is a second aspect of the present invention to provide a method in which an order sorting filter is not required to separate a light of a selected order from the rest of the unwanted overlapping orders.